1. Field of The Invention
This invention generally relates to a laser medium and more particularly to a laser medium for use in a composite slab type laser (hereunder referred to simply as a composite type slab laser medium) which can weaken amplified spontaneous emission (hereinafter abbreviated as ASE) and suppress parasitic oscillation to thereby increase an oscillation efficiency or an amplification efficiency.
2. Description of The Related Art
As a conventional solid state laser medium, is publicly known a slab laser medium which has a slab structure provided with two parallel planes facing each other as reflecting inner surfaces (hereunder referred to simply as reflecting surfaces) as disclosed in, for example, Japanese Patent Application Publication No. 48-15599 Official Gazette. This conventional slab laser medium is used to perform laser oscillation or optical amplification by extracting a laser beam therefrom. Further, in this conventional slab laser medium, the laser beam follows a zigzag path undergoing internal reflection at the alternate reflecting surfaces. Therefore, even if the distance between the reflecting surfaces is short, the optical path followed by the laser beam can be sufficiently long. In other words, even if the laser medium is made thin, a desired path length can be obtained. Thereby, the laser medium can be efficiently cooled. Thus, large pump energy can be supplied to the laser medium. This realizes laser oscillation providing a large laser output.
Further, in general, where a thermal gradient is presented within a laser medium, thermal lensing and thermal birefringence occurring due to thermally induced distortion and stress cause phase differences among laser beams to be extracted. This results in degradation of beam quality. However, in case of this conventional slab laser medium, the laser beam goes along the zigzag path between the reflecting surfaces as described above. Thus, the laser beam equally and repeatedly travels obliquely to a transverse direction, in which the thermal gradient is presented, perpendicular to the two reflecting surfaces. Consequently, the phase difference due to unevenness of refractive index in the laser medium, which is caused by the thermal lensing and the thermal birefringence, is substantially cancelled, and further a laser beam with relatively good beam quality can be obtained.
Further, in order to obtain a larger laser output and good beam quality, it is favourable for such a slab laser to have the thinnest possible laser medium. On the other hand, such a slab laser has a minimum thickness required to maintain prescribed mechanical strength and accuracy of finish of the reflecting surfaces to be formed in such a fashion to be in parallel with and face each other. Thus, there is a lower limit to thickness of the conventional slab laser which can be realized by using ordinary methods.
As a conventional laser medium obtained by making better use of the characteristic of this slab laser medium to improve beam quality and lower the lower limit of thickness, is publicly known what is called a composite slab type laser medium proposed by J. L. Emmett et al (see The Potential of High-Average-Power Solid State Lasers UCRL-53571, Lawrence Livermore National Laboratory, California, 1984). This composite slab type laser medium is devised to make a thermal gradient therein very small by including a laser activating material only in a specific region between the reflecting surfaces and moreover making the layer including the region containing the activating material very thin. Generally, in a slab laser medium, temperature is high in a central portion in the transverse direction between the two reflecting surfaces. Further, the closer to end portions (i.e., to the reflecting surfaces) a portion, the lower temperature. Thus, by removing the laser activating material from the central portion, generation of heat therein is prevented. Moreover, by making laser pumping regions of the end portions extremely thin, the thermal gradient in the transverse direction is made to be very small.
FIG. 2 is a perspective view showing the construction of a conventional composite slab type laser medium. In this figure, reference numeral 100 designates the conventional composite slab type laser medium; 100a an incident surface; 100b an exit surface; 100c and 100d reflecting surfaces facing each other; 101 a substrate portion forming an inactive layer; and 102 and 103 laser glass plate portions forming an active layer. The incident and exit surfaces 100a and 100b are formed in such a manner to be inclined at a predetermined angle, which meets Brewster's condition, away from the reflecting surfaces 100c and 100d when a laser beam is incident on the surfaces 100c and 100d in the direction parallel to the reflecting surfaces 100a and 100b. These laser glass plate portions 102 and 103 contain a laser activating material. In contrast, the substrate portion 101 does not contain any laser activating material. A mirror (not shown) to be used for causing optical resonance is placed at both ends of the laser medium in the longitudinal direction. Thereafter, when the laser medium 100 is pumped by an external pump source (not shown), is generated a laser beam which resonates in the laser medium (hereunder sometimes referred to as laser resonance light) which follows a zigzag path undergoing internal reflection at the alternate reflecting surfaces. Thus, laser oscillation is performed. In this case, a laser pumping is effected only in the laser glass plate portions 102 and 103 and namely is not performed in the substrate portion 101. As a result, in the laser medium 100, the rise of temperature is suppressed and a temperature distribution becomes uniform in the transverse direction.
Further, results of performance tests of a composite slab type laser, of which the laser medium is manufactured for trial by inventors of the instant invention, reveal that when the pump energy applied to the slab laser is less than a predetermined value, it is favourable to employ the thinnest possible glass plates, which contain the most possible laser activating material, as the laser glass plate portions 102 and 103 and that when the pump energy is increased and becomes equal to or larger than a certain value, a gain of the laser is driven into saturation. As a result of further study, it is found that the latter phenomenon is caused by the ASE and the parasitic oscillation effected in the inside of the laser medium. Incidentally, the ASE is light emitted, which is stimulated and amplified by fluorescence in a laser medium and attenuates energy stored prior to normal laser oscillation and optical amplification. Further, the parasitic oscillation is a phenomenon that in a laser medium, the ASE goes along an optical path other than a normal optical path to be followed by a laser beam which resonates in the laser medium (hereunder sometimes referred to as a resonant optical path) but perform a harmful oscillation by forming a closed resonant optical path. Further, when the ASE and the parasitic oscillation occur, the stored energy is spent for the ASE and the parasitic oscillation, so that energy of the laser beam following the normal optical path cannot be increased and consequently, a larger laser output cannot be obtained.
It has been known that the ASE and the parasitic oscillation occur in a conventional ordinary disk type laser medium and the conventional slab laser medium. Further, with respect to the ordinary slab laser medium, has been proposed a method for weakening the ASE and suppressing the parasitic oscillation.
Namely, a known method for weakening the ASE and suppressing the parasitic oscillation is what is called a segmented spacer method (see "New Slab and Solid-State Laser Technology and Application", SPIE., Vol. 736, p. 38, 1987).
FIG. 3 is a sectional view of an example of application of this segmented spacer to an ordinary slab laser medium 110. As illustrated in this figure, according to this method, gasket members 116, . . . , 116 made of rubber and so on are put into contact with outer surfaces of parts, at which a laser beam is not reflected, of the parallel reflecting planes 110c and 110d in order to prevent conditions of total internal reflection from holding. As described above, in the slab laser medium, a laser beam l.sub.1 to be extracted therefrom (hereunder sometimes referred to simply as an extraction beam) goes along a zigzag path undergoing total reflection at the alternate reflecting surfaces. As a consequence, each reflecting surface is scattered with parts of a region 115 (hereinafter referred to as a non-path region), through which the extraction beam l.sub.1 does not pass. Therefore, the efficiency of oscillation is not decreased in case where the conditions of total reflection of the laser beam l.sub.1 are made not to hold for parts of the non-path region 115. Moreover, by preventing the conditions of total reflection from holding for parts of the non-path region, reflection of light l.sub.2 generated by the ASE or the parasitic oscillation having reached the parts of the non-path region 115 can be prevented.
Further, as another method for weakening the ASE and suppressing the parasitic oscillation, is known a method disclosed in Japanese Unexamined Patent Application Publication No. 63-211779 Official Gazette. According to this method, wrapping processing is performed on portions corresponding to the parts, with which the gasket members are put into contact, of the parallel reflecting planes 110c and 110d used for effecting the segmented spacer method to form diffused reflection surfaces thereof. Thereby, parasitic oscillation can be effectively suppressed by suppressing reflection of light emitted due to parasitic oscillation (hereinafter referred to as parasitic oscillation light), which comes from the inside of the laser medium to the portions corresponding to the parts, with which the gasket members are put into contact, of the parallel reflecting planes 110c and 110d, without using gasket members and so on.
Moreover, as still another method for weakening the ASE and suppressing the parasitic oscillation, is known a method disclosed in Japanese Unexamined Patent Application Publication No. 61-287287 Official Gazette. According to this method, sandblasting processing is performed on portions corresponding to the parts, with which the gasket members are put into contact, of the parallel reflecting planes 110c and 110d used for effecting the segmented spacer method to form sandblasted surfaces thereof. Alternately, etching processing is performed on such portions to form diffused reflection surfaces thereof. Otherwise, V-shaped grooves are formed in portions corresponding to the non-path regions 115. Thereby, parasitic oscillation can be effectively suppressed by suppressing reflection of parasitic oscillation light which comes from the inside of the laser medium to the portions corresponding to the parts, with which the gasket members are put into contact, of the parallel reflecting surfaces 110c and 110d.
However, when the inventors of the instant invention applied the segmented spacer method to a composite slab type laser medium, expected effects were not obtained. According to the inventors' study of the cause, the conclusion was as follows.
The segmented spacer has been developed on the basis of an idea that reflection of a laser beam at parts of the non-path region is restrained by making the conditions of total reflection from holding for the parts of the non-path region. Thus, the gasket member 116 is used as a member for making the conditions of total reflection from holding. In other words, the ASE and the parasitic oscillation light impinge on the reflecting surface at a certain angle can be effectively made extinct by using the segmented spacer, while the segmented spacer method is ineffective against the ASE and the parasitic oscillation light go on in parallel with the reflecting surfaces. Generally, in an ordinary slab laser medium, a relatively large part of the ASE and the parasitic oscillation light impinges on the reflecting surface at a certain angle, so that the segmented spacer method is effective to a certain extent.
However, in case of the composite slab type laser medium, most part of the parasitic oscillation light advances in the laser glass plate portions 102 and 103 in parallel with the reflecting surfaces 100c and 100d. Consequently, if the segmented spacer method is applied to the composite slab type laser medium without change, expected effects cannot be obtained.
Further, the results of the experiments made by inventors of the present invention reveals that the gasket member 116 is very easily deteriorated by iteration of the laser oscillation and optical amplification. From an investigation, it is found that the cause of this is a phenomenon that the gasket member 116 is not also heated by heat conducted from the laser medium but also absorbs pumping light and light emitted due to parasitic oscillation light and generates heat and thus temperature of the gasket member 116 is liable to rise to a permissible temperature and higher. Especially, this phenomenon is conspicuously presented in case that an air-cooling method with low cooling efficiency is employed for cooling the laser medium.
Moreover, in case of the method disclosed in Japanese Unexamined Patent Application Publication No. 63-211779 Official Gazette, differently from the segmented spacer method, it is unnecessary to use gasket members having low heat resistance, and degradation of gasket members owing to heat does not occur. This method, however, has a problem of suppressing parasitic oscillation light advancing in parallel with the reflecting surfaces similarly as the segmented spacer method does. Thus, similarly as in case of the segmented spacer method, if the method disclosed in Japanese Unexamined Patent Application Publication No. 63-211779 Official Gazette is applied to the composite slab type laser medium without change, expected parasitic oscillation suppressing effects cannot be obtained. In addition, as described above, according to this method, the wrapping processing is performed on the surfaces of the laser glass plate portions. Thus, the surfaces of the laser glass portions are as good as scratched in a sense. It is well known that scratches on the surface of glass considerably decrease mechanical strength of the glass and a maximum stress, which is a cause of thermal destruction occurring when pump light is absorbed, is generated on the surface of the laser medium. Thus, this method has a drawback in that the laser glass plate portions are liable to cause thermal destruction and consequently, there is a limit to an average input of the pump light applicable to the laser medium.
Furthermore, the method disclosed in Japanese Unexamined Patent Application Publication No. 61-287287 Official Gazette is similar to the method disclosed in Japanese Unexamined Patent Application Publication No. 63-211779 Official Gazette in respect of scratching the surface of glass and vicinities thereof, and accordingly has the similar defect as the method disclosed in Japanese Unexamined Patent Application Publication No. 63-211779 Official Gazette.
The present invention is intended to obviate the above described drawbacks of the prior art.
It is therefore an object of the present invention to provide a composite slab type laser which can effectively weaken ASE and suppress parasitic oscillation and stably perform laser oscillation and light amplification for a long period of time.